title: Signaling efficacy drives the evolution of larger sexual ornaments by sexual selection.
abstract: Why are there so few small secondary sexual characters? Theoretical models predict that sexual selection should lead to reduction as often as exaggeration, and yet we mainly associate secondary sexual ornaments with exaggerated features such as the peacock's tail. We review the literature on mate choice experiments for evidence of reduced sexual traits. This shows that reduced ornamentation is effectively impossible in certain types of ornamental traits (behavioral, pheromonal, or color-based traits, and morphological ornaments for which the natural selection optimum is no trait), but that there are many examples of morphological traits that would permit reduction. Yet small sexual traits are very rarely seen. We analyze a simple mathematical model of Fisher's runaway process (the null model for sexual selection). Our analysis shows that the imbalance cannot be wholly explained by larger ornaments being less costly than smaller ornaments, nor by preferences for larger ornaments being less costly than preferences for smaller ornaments. Instead, we suggest that asymmetry in signaling efficacy limits runaway to trait exaggeration.
authors.affiliation: Theoretical Biology, Institute of Integrative Biology, ETH Zürich, Universitätstrasse 16, CH-8092, Zürich, Switzerland; CoMPLEX, University College London, Gower Street, London, WC1E 6BT, United Kingdom; The Galton Laboratory, Department of Genetics, Environment, and Evolution, University College London, Gower Street, London, WC1E 6BT, United Kingdom. samuel.tazzyman@env.ethz.ch.
authors.forename: Samuel J
authors.initials: SJ
authors.lastname: Tazzyman
authors.forename: Yoh
authors.initials: Y
authors.lastname: Iwasa
authors.forename: Andrew
authors.initials: A
authors.lastname: Pomiankowski
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D052586:Mating Preference, Animal; D008957:Models, Genetic; D012641:Selection, Genetic; D012727:Sex Characteristics; D017603:Signal Detection, Psychological
keywords: Fisher's runaway; mate choice; mate preference; sexual dimorphism; sexual ornament; sexual selection